The invention relates to the production of concrete elements having a compressive strength of not less than 400 MPa and fracture energy of not less than 1,000 J/m.sup.2, prefabricated or cast in-situ.
According to the invention, to obtain such results, a method is provided comprising the steps of mixing a composition comprising the following components expressed in parts by weight (p):
a) 100 p of Portland cement;
b) 30 p to 100 p or better 40 p to 70 p of fine sand having a grain size of at least 150 micrometers;
c) 10 p to 40 p or better 20 p to 30 p of amorphous silicon having a grain size of less than 0.5 micrometers;
d) 20 p to 60 p or better 30 p to 50 p of ground quartz having a grain size of less than 10 micrometers;
e) 25 p to 100 p, or better 45 p to 80 p of steel wool;
f) a dispersing agent;
g) 13 p to 26 p or better 15 p to 22 p of water; and after setting, curing the concrete at a temperature of 250.degree. C. or higher, for a length of time sufficient to transform cement hydration products into crystalline hydrates of the xonotlite type; thus eliminating substantially all of the free water and at least the main part of the adsorbed and chemically bonded water.
The use of sand aggregates, specially of siliceous sand, provides a high bond strength at the interface aggregates/past of cement, silica and water.
It is known that concrete can be cured in an autoclave to transform amorphous hydrates of concrete of the [CaO SiO.sub.2, H.sub.2 O] type into a crystalline hydrate known as tobermorite whose formula Is [CaO].sub.5 [SiO.sub.2 ].sub.6, [H.sub.2 O].sub.5, however that technique does not constitute a satisfactory solution to the present problem since tobermorite includes five times more water than xonotlite whose formula is [CaO].sub.6 [SiO.sub.2 ].sub.6 [H.sub.2 O].sub.1.
Preferably the transformation of the hydration product of the cement is obtained merely by heating, under ambient conditions of pressure and humidity.
In a particular embodiment of the invention, a concrete is prepared having an accumulated porosity of less than 0.01 cm.sup.3 /gram (as measured using a mercury porosimeter), containing ground quartz and steel wool, constituted by ground steel shavings and is cured after setting to a temperature not less than 250.degree. C., and preferably of not less than 400.degree. C., under ambient conditions of pressure and humidity, for a length of time sufficient to obtain and transform cement hydration products into crystalline hydrates of the xonotlite type.
The water vapor generated during heating remains confined within the concrete because of its low porosity. The ground quartz facilitates the formation of crystalline hydrates that are richer in CaO than the amorphous hydrates, and the steel wool impart sufficient strength to the matrix during the transitional stage when the water vapor pressure in the pores is at a maximum.
Under such conditions, the entrapped water vapor achieves the hydrothermal conditions within the concrete that are required for transforming amorphous or semi-crystalline hydrates into crystals of xonotlite.
Curing generally lasts for a period of several hours.
A typical sequence for hot curing is given in FIG. 1. The ratio of residual water to initial water is plotted on the vertical axis. It can be seen than it takes a long time to reach constant weight at a given temperature. In practice, the curing sequence can be accelerated when higher temperatures are used.
Preferably, the grain size of the fine sand is less than 800 micrometers, and more preferably in the range 150-400 micrometers.
Preferably, at least 0.6 p, or better at least 1.4 p of a super plasticizer is added to the composition.
Preferably steel wool is made of steel shavings ground to 1 mm to 5 mm (size of the cutting grid of the grinder).
Preferably Portland cement is of type Vor III or a high silica modulus cement.
Preferably the concrete is pressurized before and during setting at a pressure in the range 5 MPa-50MPa.
According to the invention, an addition to the composition of 1 to 4%, preferably 2 to 3%, (in volume with respect to the volume of the concrete after setting) of metal fibers having a diameter smaller than about 500 micrometers, preferably An the range 100-200 micrometers, a length in the range of 4 mm-20 mm, preferably 10 mm-14 mm, improves considerably the flexural strength and the fracture energy.
The invention will be illustrated hereafter with some typical examples.